In recent years, automobiles and their parts are being produced in various places over the world such as China, Korea, Southeast Asia, India, Near East, North America, Canada, etc. The coating process in an automobile manufacturing line consists of under coating, middle coating, and top coating processes, and generally an electrodeposition coating is adopted as the under coating process.
In a coating equipment used in an electrodeposition coating process, a whole automobile body is immersed in an electrodeposition cistern containing electrodeposition paint and a voltage is applied to make the paint precipitate on the surface of the automobile body. And thereafter the body is washed and then baked in a drying furnace to form an electrodeposition layer. Since a large amount of paint is used in an electrodeposition coating process, an automatic coating and/or automatic paint feeding equipments are introduced. Furthermore, since there are many items to be managed, generally a control system for managing the coating equipment based on the measured data in the coating process is introduced.
FIG. 6 is a block diagram to show a configuration of typical electrodeposition coating equipment. The electrodeposition coating equipment includes an electrodeposition cistern (602) to accept the automobile body, a first reclaim washing cistern (604), a second reclaim washing cistern (605), an industrial water washing cistern (606), and a pure water washing cistern (607).
In the electrodeposition coating process, an automobile body (613) is carried by a carrier (612) in a suspended state to be immersed in the electrodeposition cistern (602) having a capacity of 10 to 300 tons and filled with electrodeposition paint. A coating layer is precipitated on the surface of the automobile body by applying a voltage of 250 to 300 V between the automobile body, which acts as an electrode, and an electrode plate (603) from a rectifier (601). Thereafter, the automobile body is washed in the first and second reclaim washing cisterns (604, 605), the industrial water washing cistern (606), and the pure water washing cistern (607). The cleaning fluid used in the washing processes at the first and second reclaim washing cisterns (604, 605) is a filtrate which is obtained by filtering the paint composition under pressure using an UF (Ultra Filtration) film. The filtrate is temporarily stored in a filtrate tank (611) and thereafter delivered to each washing cistern by a pump (not shown). The pigment components remained after the filtration are returned to a sub tank (610).
The paint which flows from the electrodeposition cistern (602) to the sub tank (610) serves to remove the foam generated at the surface of the paint in the electrodeposition cistern (602) since the sub tank (610) is provided with a liquid level difference, thereby preventing the foam from adhering to the automobile body (613). The paint collected from the first and second reclaim washing cisterns (604, 605), the precision filtration apparatus (608), and the UF module (609) is returned to the electrodeposition cistern (602) through the sub tank (610).
In the washing processes by use of the industrial water washing cistern (606) and the pure water washing cistern (607), industrial water and pure water are used as the cleaning liquid respectively. In the process (not shown) following these washing processes, the automobile body is dried by baking at a temperature of 150° C. to 200° C. for 10 to 90 minutes in a drying furnace to obtain an under coating layer.
Generally, the electrodeposition paint used in the electrodeposition coating process is a cationic electrodeposition paint in view of corrosion resistance, which consists of: an emulsion in which amino added epoxy resin as a base resin, a blocked isocyanate hardener, other additives, and acids of neutralizing agents are dispersed with water; and pigment paste in which coloring pigments, rust preventive pigments, and other pigments are dispersed with a dispersion resin.
In order to achieve a layer thickness, a finishing property (i.e. a uniformity of the coating surface), and a deposition property (i.e. a state of coating in narrow gaps) of the intended levels in an electrodeposition process, many check items regarding the electrodeposition paint are controlled in a timely fashion, which are such as pH, electric conductance, concentration of solid content, ash content, acid concentration, coat thickness of various parts of an automobile body, state of coating surface (i.e. generation of pinholes, etc.), deposition property, and others.
There are also many control items regarding the electrodeposition coating equipment, which are coating voltage of a rectifier (601); current value appearing during electrodeposition; liquid level, flow rate, and level difference from the sub tank (610) of the coating liquid in the electrodeposition cistern (602); current value in each electrode plate (603); concentration of solid content and pressure difference through filters in the first and second reclaim liquids; flushing pressure (water pressure at the nozzle) and state of the nozzles in the pure water washing process; pressure difference through filters and replacement frequency of the filters of the precision filtration apparatus (608); amount of permeation through the UF module (609) and replacement frequency of the module; and others.
As described above, to maintain a good finishing condition of the coating in an automated coating process, it is necessary to adjust the coating conditions (the voltage, cistern temperature, electrification time, etc. in the electrodeposition coating) adapting for the state of the paint. Furthermore, it is necessary to do sampling of the paint being used 1 to 3 times a week to determine the paint properties such as pH, electrical conductivity, acid concentration, coat thickness, etc. by analysis tests and thereby to perform, based on the estimation by the engineer, the determination of the amount and timing for adding the neutralizing agent, water, organic solvents, etc.; the control of coating conditions; the adjustment by supplementary paint; and the like.
For making these adjustments, expert engineers' knowledge is essential. There may be cases where no such expert engineer stays constantly at the coating line of a factory at a remote site in the home area and at overseas. In such cases, the coating line is managed in the following way.
The paint in use is sampled at a remote factory and is transported to a managing department by use of road, railway, air, or sea transportation so that the received paint is analyzed at the managing department. The data on the coating conditions regarding the coating equipment and the paint properties are recorded in a check sheet by the operator on the site and the check sheet is sent to the engineer of the managing department by facsimile. The engineer at the managing department scrutinizes the analysis results and the data on the coating conditions sent from the remote site and discuss the results with the on-the-site operators by way of telephone or the like to give instructions about the coating equipment.
As described above, in the coating line management based on instructions from the line control department to an on-the-site factory, problems exist in that it takes time to transport the sampled paint and, in addition to that, since the pigments tend to settle by being separated from the solvent during the transportation, it would take more time to return the paint into the original condition for analysis at the managing department. There is also another problem in that when the sampled paint was not sufficiently returned to its original state, the variation of test samples taken from the sampled paint would cause an increase in the measurement error in the analysis.
From these reasons, there were cases where the adjustment of the coating conditions was delayed or the instruction on the amount of the additives to be introduced in the paint was inaccurate.
As the measure for these problems, one possible way may be the placement of an engineer at each on-the-site factory, but it is costly to have engineers stationed at remote sites, especially at sparsely dispersed overseas sites, which has been a factor to hinder the deployment of providing paints to remote sites, especially to overseas automobile production sites.